Screw propeller



July 7, 1925.

A. H. R. FEDDEN ET AL SCREW PROPELLER 3 Sheets-Sheet 2 July 7, 1925. 1,545,416

A. H. R. FEDDEN ET AL scnEw P'ROPELLER Filed Nov. 21. 1924 s Sheets-Sheet 5 a m; M wwat,

Patented July 7, 1925.

UNITED STATES PATENT OFFICE.

ALFRED HUBERT ROY FEDDEN AND LEONARD FREDERICK GEORGE BUTLER, OF BRIS- TOL, ENGLAND, ASSIGNORS TO THE BRISTOL BRISTOL, ENGLAND, A. BRITISH. COMPANY.

.AEROPLANE COMPANY LIMITED,

SCREW PROPELLER.

Application filed November 21, 1924. Serial No. 751,308.

To all whom it may concern: v

Be it. known that we, AL'rnEn HUn R'r ROY FEnnEN and LEONARD FREDERICK GEORGE BUTLER, both subjects of the King of England, and both residing at Bristol, in the county of Gloucester, England, have invented certain new and useful Improvements in Screw Propellers, of which the following is a specification.

This invention is for improvements in or relating to screw-propellers of the type known as feathering or variable pitch propellers, in which the blades are capable of angular adjustment each about its own longitudinal axis, that is to say, about a line extending from the root to the tip of the blade, for the purpose of varying the effective pitch of the propeller. The invention is particularly applicable to propellers 90 or air-screws, such as are used in aircraft,

but it is not limited to this particular application of it.

According to this invention there is pro vided in a variable-pitch propeller, the combination of a driving-shaft, two or more, blades carried thereby, and so mounted as to be capable of angular adjustment each about its own longitudinal axis, an operating-shaft formed with a sere.wthread whereof the angle, for the purpose later set forth, is small, a nut-member on said operating-shaft operativcly engaging the said blades to turn them each about its own axis by a movement of the nut-member along the said shaft lengthwise thereof, and means for producing relative rotation between the nutmember and the operating-shaft to produce the said traversing movement.

The invention also comprises a variablepitch propeller wherein the operating-shaft is constituted by a screw-threaded portion of or on the driving-shaft, and a nut-member thereon carries a sleeve which receives the axial movement but not the rotational movement of the nut-member, and also is .operatively engaged with the blades to vary their effective pitch by the said axial movement.

Furthermore, the invention comprises in a variable-pitch propeller, the combination with the nut-member of means, such as a hunting friction-gear, for operatively connecting it to the engine which drives the propeller to effect the changing of the p1tch.

According to another feature of this invention the means for rotating the nut-membcr may comprise a gear-wheel fast on it engaging a gear-wheel driven 'by a frictionwhcel which is adapted to co-opcrate with a normally stationary part of the engine, with or without, a speed-reducing gear between the friction-wheel and the second-mcntioned gear-wheel.

More specifically this invention comprises in a variable-pitch propeller, the combination of two rotatable shafts carried by the sleeve aforesaid so as to be moved axially therewith, and situated ceccntrically with respect to the driving-shaft, a friction-wheel on each of said shafts, a (to-operating friction surface or surfaces on a non-rotating part of the engine adapted to be engaged alternatively with said friction-wheels by movement axially of the driving-shaft, a gear-wheel on each of said shafts engaging respectively internal and external gears formed on a flange-like member encircling and rotatable on the sleeve, and a double planetary gear carried by said member engaging respectively a gear fast on the sleeve and a gear fast on the nut-member.

Another feature of this invention consists in the arrangement of the gearing aforesaid whereby, for the p'urpose hereinafter set forth, the movement of the friction-surface aforesaid in one or other direction along the axis of the driving-shaft to set the gear inoperation causes the nut-member to be traversed along the driving-shaft in the same direction.

The invention also comprises a variablepitch propeller as above set forth whereinthe friction-surface aforesaid is for the purpose hereinafter described a segment only of a circle; furthermore, the friction-surface may be resiliently mounted, for the purposehereinafter described.

Other features of the invention relating to the control will be hereinafter described and the novel details pointed out in the claims appended to this specification.

In the accompanying drawings- Figure 1 is a longitudinal section of the hub of 'a screw-propeller constructed in accordance with this invention, being a section on the line 11 of Figure 2;

Figure 2 is a section on the line 2-2 of Figure 1;

Figure 3 is a part-longitudinal section of a hub showing a modified construction;

Figure 4 is a sectional view of a detail on the line 4-4 of Figure 3; and

Figure 5 is a diagrammatic representation of the pilots controlling gear for the is indicated at 10 and an extension 11 of the crank-shaft constitutes the driving-shaft of the propeller. Upon this shaft 11 there is mounted a hub comprising a sleeve or cylindrical portion 12 and a radial flange 13, and this hub is locked to the shaft 11 to rotate with it. The sleeve portion 12 of the hub is formed on the side of the flange remote from the engine'with an irreversible screw-thread 14, that is to say with a thread having a small spiral angle; conveniently this thread is of acme form, although other forms may be used if so desired, and in order to keep! the spiral angle small, the thread 14 is preferably a single-start thread.

Upon this screw-threaded sleeve 12, which constitutes the operating shaft for varying the pitch of the propeller. there is mounted a nut-member 15 which takes the form of a sleeve internally-screw-threaded as at 16 to co-operate with the thread 14 aforesaid. The nut-member 15 is axially of less length than the screw-threaded portion of the hub 12 whereon it is mounted, and it can be traversed along itby producing relative rotation between them; On the outer cylindrical surface of the nut-member 15 there is mounted a sleeve 17 which is held against endwise movement on the nutmember between a flange 18 thereon and a nut 19, so that it partakes of the endwise or axial movement ofthe nut-member but does not partake of the rotation of the nutmember when it is screwed along the hub 12, the nut-member15 turning freely within the sleeve 17.

A cylindrical casing 20* surrounds the nutmember 15 and sleeve 17 aforesaid and is gripped between the flange 13 and a detachable flange 21 which is screwed on to the outer free end of the hub 12; thiscasing 20 is preferably also bolted to-the flanges 13, 21, so that a positive drive is imparted to it when the flan es rotate.

The sleeve 17 is formed at about the middle of its length with a radially-extending flange 22, and a similar flange-23 is provided at the end near the engine. A tubular member 24 is suitably secured in perforations in the flanges and it extends, to the such manner as topartake of the endwise movement of the sleeve 17, and this flangelike member is provided with two sets of gear-teeth, an internal set 33 and an external set 34. The internal set .of teeth 33 mesh with the gear-Wheel 31 aforesaid, whilst the external teeth 34 mesh with the companion gear-Wheel 30.aforesaid. This arrangement provides that if one or other of the shafts 27 is rotated in one direction, the annular flange-like member 32 is rotated in one or other direction on the sleeve This member 32 also carries one or more double planetary pinions 35, 36 which mesh respectively with a gear-wheel 37 fast on the .sleeve 17 and a gear-wheel 38 fast on the nut-member 15. The whole thereby constitutes an epicyclic train of gearing whereby rotation of the flange-member 32 in one direction or other imparts relative rotation to the sleeve 17 and the nut-member 15. The sleeve-member 17 cannot rotate relatively to the flange 13 and hub 12, and the relative movement aforesaid is therefore a relative movement between the nut-member 15 and the hub 12, so that the nut-member is traversed along the hub 12 in one direction or the other according to the direction of rotation of the flange-member 32.

. In order to provide for driving one or other of the shafts 27 aforesaid, each of these shafts carries at the end near the engine a friction-wheel 39, 40 respect1vely,- these wheels lying within the extension 25 of the crank-case aforesaid. A friction-surface 41 to eo-operate with these wheels is arranged inside the part 25 aforesaid and is movable lengthwise of the shaft. Conveniently a single friction-surface 41 in c lindrical form is provided and the two w eels 39, 40 are displaced lengthwise of the dIlV- ing-shaft 11 so as to lie one on each side of it. The friction-surface 41 is movable endwise to engage one or other of these wheels, that is to say in Figure 1 if it is moved towards the right it engages the wheel 39, and if it is moved towards the left it engages the wheel 40. It will be appreciated that when the shaft 11 is in rotation the whole of the mech'anism'is carried round with it, including the wheels 39, 40, and by their engagement with the statlonar friction-surface 41 a. drive is imparted a ternatively to them, the direction of the drive being the same in both cases.

The train of gearing, consisting of the friction-surface 41, friction-wheel 39 (or 40), gear-wheels 30, 34 (or 31, 33), 35, 37, 36, 38, is so arranged that when the frictionsurface 41 is moved to engage one or other of the friction-wheels 39, 40, the resulting axial movement which is given to the nutmember 15 is such that it is traversed axially in the same direction as the friction-surface 41 was originally moved; this movement will continue until the corresponding friction-wheel is carried just clear of the friction-surface. That is to say, assume the friction-surface is moved to the left in Figure 1', it will engage the wheel 40 and set the gear in motion so that the nut-member 15 is traversed to the left, and this traversing movement continues until the wheel 40 is carried just clear of the frictionsurface 41. Further, it will be appreciated that the wheel 39 partakes of the same axial movement as the wheel 40, so that in the final position the relative setting of the friction-wheels'and the friction-surface is the same as in the original position. This, of course, necessitates that the traversing movement of the nut-member 15 is exactly equal to the movement imparted to the friction-surface, 41.

It will be appreciated that the drive is imparted to one or other of the wheels 39, 40 by their engagement with the frictionsurface 41, the wheels being carried around by the rotation of the shaft 11-. The power required for traversing the nut-member is therefore derived in the first instance from the engine which turns the-shaft 11, and the amount of traversing movement is controlled manually by the longitudinal movement of the friction-surface 41..

Turning now to Figure 2 which, as above mentioned, is a sectional view on the line 2-2 of Figure 1, the flange 22 aforesaid on the sleeve 17 has formed on it a series of lugs 42, one for each of the blades of the propeller. These lugs are each bored to receive a hardened steel pin 43 which is situated with its axis transverse to that of the driving-shaft 11. Each of these pins carries at one end a square bush 44 which is received in a straight-sided U-shaped slot 45 so that it is free to move therein, this movement in Figure 2 being in a vertical direcv tion. This slot is formed on the end of an extent. sleeve 17 and flange 22 is obtained in the ball-bearing of the cup-and-cone type, which permits free rotation of the socket 49 and also providesefi'ectively for absorbing any end-thrust on the blade due to centrifugal force whilst the propeller is in rotation. This bearing is constituted by the ring 47 aforesaid and a second ring 50, these being secured respectively on the socket 49 and on the casing 20.

The radially outer bearing for the socket is indicated at 51, and it may be a rollerbearing or a plain sleeve bearing, its function being merely to act asan additional guide for the socket.

In the operation of this device, axial movement of the sleeve 17 and its flange 22 produces a rotational movement of each propeller blade about its longitudinal axis, owing to the engagement of the pin 43 with the slot on the end of the arm 46. The pin 43 has a straight-'linemovement normal to the surface of the paper in Figure 2, and the arm 46 has, of course, a circular, movement about the axis of'the propeller blade. The construction of bush 44 and slot 45, however, permits the desired movement to take place, it being appreciated that the angular movement of the propeller blade about its own axis is not required to be of very great The endwise movement of the manner above described by traversing the nut-member 15 along the hub 12, and it will be appreciated that by using an irreversible thread 14 of small spiral angle, the mechanism is self-locking in any position, that is to say the angular setting of the blades cannot be altered by any pressure or force applied directly to them.. Thevwhole of the thrust on the blades, which is due to their setting or obliquity and which tends to rotate them about. their respective longitudinal axes, is taken up on the screw-thread 14, and there is no transmission of this thrust to the mechanism whereby the pitch of the blades 1s adjusted. ,The pitch of the propeller can therefore be altered only 'by rotating the nut-member 15 onthe hub 12, and this operationis controlled only by the pilot of the aircraft (by adjustment of the frictionnormally rotates at high speeds, and any adjustment of the pitch of the blades should be made only gradually; itis for this reason, therefore, that the epicyclic train of gears is introduced in the construction illustrated in Figure 1. Figure 3 illustrates a modified construction whereby the necessary speed reduction can be obtalned. In

which corresponds to the member 32 of Figuse 1, is keyed directly to the nut-member 15, and it carries internal and external gears this case the annular flange-like member 52,

as before. These gears are driven as before by the pinions 30, 31, ofwhich only one is shown, and the pinions themselves are driven by the friction-wheels 39, 40 also in the same way. Instead of providing a friction-surface which is in the form of a complete cylinder, however, it is of segmental form, as shown at 53. By this means the friction-wheels engage it only during a small part of each rotation of the drivingshaft 11. The traversing movement imparted to the nut-member is of course intermittent, but the desired speed reduction is obtained without the use of gearing. The friction-surface 53 is preferably resiliently mounted, as shown in Figures 3 and 4, be-

ing provided with a spring 54 which normally presses it radially inward and allows it to yield outwards when the friction-wheels come in contact with it. The surface which the wheels engage may be later curved to a larger radius than the circle on which the outermost part of the friction-wheels travels. The shock of the impact of the friction-v wheels on it is thereby reduced, and the drive is gradually taken up by these wheels whilst they are in contact with it. Obviously, in stead of mounting the member 53 resiliently, the bearing near the wheel 39 could be resiliently mounted, or, alternatively, the

friction-wheel itself may be provided with a suitable resilient peripheral member, such for example as rubber or spring steel.

This friction-member 53 is controlled in the same way as the part 41 of Figure 1, and is illustrated as being moved endwise by a controlling handle 55 against the action of a spring 56, but it will be appreciated that these details may be modified in any desired manner. The arrangement of the frictionsurface 41 or 53, with its two cooperating friction-wheels 39, 40, constitutes a hunting gear, and it will be appreciated that various other devices may be used, including a servo-motor, to obtain a similar result.

The other details of the construction illustrated in Figure 3 are similar to the construction illustrated'in Figure 1 and need not be described again in detail.

Instead of using a drive through frictionwheels as above described for effecting the adjustment of the propeller-blades, the mechanism illustrated in Figures 6, 7 and 8 may be used. Figure 6 is a view corresponding to the right-hand part of Figure 1, Figure 7 is a planpartly in section of Figure 6, and Figure 8 is 'a section on the line 88 of Figure 7. The tubular member 24 in Figure 6 corresponds with the member 24 in Figure 1, as also does the shaft 27. This shaft is mounted to rotate, in Figure 6, in aball-bearing 66 mounted in a suitable housing 67 which is secured to the tubular member 24 by means of a sleeve 68. On the end of the shaft 27 there is mounted a gear-wheel 69 which meshes with an internally-toothed ring 70. The corresponding shaft 27 in the tubular member 24 at the lower part of the figure is similarly mounted and carries a gear-wheel 71 which meshes with a second internally-toothed ring 72. These two rings are mounted on roller-bearings 73 which are supported in suitable flanges 74 on the propeller hub, so that whilst the rings 70, 72 normally rotate with the propeller hub either of them can be retarded or held so as to rotate relatively to the propeller hub.

The two rings 70, 72 are provided with outwardly-extending flanges which face one another and carry on their opposed faces a lining of a suitable friction material 75, 76.

Turning now to Figures 7 and 8, it will be seen that the brake-shoes 77 are mounted between the two friction-surfaces 75, 76 in such manner that they can be engaged with either one of them; the brake-shoes 77 are mounted upon levers 78 which are free to swing on a shaft 79. As shown in Figure 7, each lever 78 isprovided with an upward extension 80 which lies within a housing 81, and compression-springs 82, 83 engage opposite sides of this extension 80 through thimbles 84, 85 respectively. A pin 86 is secured in the housing 81 between tail-pieces on the thimbles 84, 85 so that it will engage one or other of these tail-pieces when relative movement takes place between the housing 81 and the part 80 of the lever 78. One of the housings 81 is provided with. an upward extension 87 which constitutes the controlling lever for the mechanism corresponding to the controlling lever 55 of Figure 3. This lever 87 with the housings 81 is fast on the shaft 79 aforesaid, so that movement of the lever 87 rocks both housings.

The operation of this mechanism is as follows. Suppose. that the upper end of the lever 87 is moved to the right in Figure 7, it rocks the housings 81 and each spring 82 is therefore compressed and exerts a pressure on the thimble 84 and through it on the part 80 of the lever 78. Simultaneously each pin 86 engages the tail-piece on the thimlble 85 and moves the left-hand end of the spring 83 towards the right whilst the right-hand end of the spring is moving towards the right with the housing 81. The pressure of the spring 83 on the part 80 of the lever 78 is thereby entirely relieved. The net result of this movement of the lever'87 is that pressure is exerted upon the lever 78 to move it in such a direction that the brake-shoes 77 are pressed against the friction-surface 75 of the ring 70 and thereby retard its mov ment. A drive is thereby impartedlto the gear-wheel 69 whilst the" companion gearwheel 71 is left free and the blades of the propeller will be adjusted accordingly in the manner hereinbefore described. This adjustment carries all ,the gearing and the rings 70, 72 bodily, axially of the hub; this movement continues so long as the spring 82 exerts any pressure on the part 80 of the lever 78 and therefore depends upon the extent of the initial movement of the lever 87. As soon as the lever 78 has moved .into a position in line with the lever 87, the

two springs 82, 83 become balanced and therefore inoperative and the systemis in equilibrium again and the brake-shoes 77 are not pressed upon the friction surface 75 of the ring and the motion therefore stops.

A reverse movement of the lever 87 causes the ring '72 to be retarded so that the drive is imparted through the gear-wheel 71 to the adjusting mechanism of' the propeller blades in the reverse direction. It will be seen therefore that this mechanism is exactly similar in its operation to the mecha nisms previously described.

One of the factors to be considered when selecting the pitch to which a propeller. or air-screw on an aircraft should be adjusted, is the density of the atmosphere wherein it is working; it will be appreciated that the principal variation in density arises from variationsin the height of the aircraft, and it is important that the pitch should never be reduced to such a degree as will permit the speed of the engine to increase to a dangerous amount. Moreover, there will usu ally be a maximum effective pitch at which the highest efliciency of the engine will be obtained, and these minimum and maximum values will vary with the density of the atmosphere. i

The question of determining the permissible pitch for the propeller is of the greatest importance when engines of the super charging type are used, that is to say, engines in which the pressure of the air supplied to the carburetor is maintained by a.

blower at the normal pressure existingat ground level, independently of the altitude of the aircraft. The power developed by the engine does not diminish as the density of the air in which the propeller is working diminishes, and it is therefore essential to increase the effective pitch of the propeller when the engine is run at full throttle at high altitudes.

As above stated, the pitch of the propeller is changed by means of power drawn from the engine under the manual control of the pilot, the operation effected by the pilot being the longitudinal movement of the friction-surface aforesaid, and this movement is conveniently effected .by means of a small control-lever. This lever will be provided with a suitable indicator or scale whereon the maximum and minimum permissible pitches are shown. An-

movement determines the variation in pitch of the propeller, of an indicator showing the maximum and minimum permissible pitch and means for automatically adjusting said indicator in accordance with the power developed by the engine, and/or the density of the medium. in which the .propeller is working.

More specifically this invention comprises the combination of a control-lever swinging over an indicator or scale, a flexible diaphragm subjected to atmospheric pressure on one side and on the other side to the pressure of the induction system of an internal-combustion' engine driving the pro-' peller, and an operative connection whereby the movements of the said diaphragm impart to the indicator a movement or adjustment relatively to the control-lever.

Fi ure 5 illustrates a simple embodiment of this feature. The pilots control lever 57 is pivotally mounted at 5'8 and connected by. a rod 59 directly or indirectly to the friction-surface aforesaid, being, for example, connected to the lever 55 of Figure 3. A segmental plate 60 is mounted to turn on the spindle 58, and it is o ratively connect-v ed by a pinion 61 and me 62 to a pressureoperated device, such as a flexible dia-"- in atmospheric pressure causes the diaphra m 63 to move upwards in Figure 5 and there y turns the pinion 61 and the segment 60 in an anticlockwise direction. The max mum permissible pitch of the propeller s increased under these conditions and this increase is shown by the-movement of the maximum mark in the direction in which the control-lever 57 is moved for increasing the pitch. Simultaneously, of course, a decrease in the atmospheric density raises the minimum permissible value of the pitch, since the speed at which the engine can safely run is limited, and this limitatlon is also provided for by the simultaneous movement of the .minimum mark on the segment 60. Obviously, this arrangement may sire to secure by Letters Patent is 1. In a variable-pitch propeller, the combination of a driving-shaft, a plurality of blades carried thereby and so mounted as to be capable of annular adjustment each about its own longitudinal axis, an operating shaft, a screw-thread of small spiral angle formed thereon, a nut-member on said screw-thread, means connecting said nutmember to said blades to turn them each about its own axis by a movement of the nut-member lengthwise of the operatingshaft, and means for producing relative rotation between said nut-member. and said operating-shaft to produce said lengthwise movement.

2. In a variable-pitch propeller, the combination of a driving-shaft, a plurality of blades carried thereby and so mounted as to be capable of an ular adjustment each about its own longitudinal axis, a screw-threaded part on said driving-shaft, a nut-member on said screw-threaded part, a sleeve rotatably mounted on said nut-member, means holding said sleeve against rotation with said nutmember but permitting its axial movement therewith, means connecting said sleeve to said blades to impart angular movement thereto each about its own longitudinal axis by lengthwise movement of the said sleeve along said driving-shaft.

3. In a variable-pitch propeller, the combination of a driving-shaft, a plurality of blades carried thereby and so mounted as to be capable of angular adjustment each about its own longitudinal axis, a part on said driving shaft having a screw-thread of small spiral angle formed on it, a nut-member on said screw-thread, means connecting said nut-member to said blades to turn them each about its own axis by a movement of the nut-member lengthwise of the said screwthreaded part, a power-unit for rotating said driving-shaft, means operable'at will for connecting said nut-member to said power-unit to produce relative rotation between the nut-member-and the screw-threaded part engaged by it.

4. In a variable-pitch propeller, the combination of a driving-shaft, a plurality of blades carried thereby and so mounted as to be capable of angular adjustment each about its own longitudinal axis, a part on said shaft having a screw-thread of small spiral angle formed on it, a nut-member on said screw-thread, means connecting said nutmember to said blades to turn them each about its own axis by a movement of the nut-member lengthwise of the operatingshaft, a power-unit for rotating said driving-shaft, and a hunting friction-gear operable at will for operatively connecting said nut-member to said power unit to produce relative rotation between the nut-member and the screw-threaded shaft engaged by it.

5. In a variable-pitch propeller, the combination of a driving-shaft, a plurality of blades carried thereby and so mounted as to be capable of angular adjustment each about its own longitudinal axis, a art on said shaft having a screw-thread of small spiral angle formed on it, a nut-member on said screw-thread, means connecting said nutmember to said blades to turn them each about its own axis by a movement of the nutmember lengthwise of the operating-shaft,

a power-unit for rotating said driving-shaft,

bination of a driving-shaft, a plurality of blades carried thereby and so mounted as to be capable of angular adjustment each about its own longitudinal axis, a part on said shaft having a screw-thread of small spiral angle formed on it, a nut-member on said screw-thread, means connecting said.

nut-member to said blades to turn them each about its own axis by a movement of the nut-member lengthwise of the operatingshaft, a power-unit for rotating said driving-shaft, a gear-wheel fast on said nut- .member, a friction-wheel having a rotary movement relatively to said power-unit, a speed-reducing gear rotatively connecting said friction-wheel with said gear-wheel, and a friction-element carried by said power unit and movable at will into engagement with said friction-wheel.

7. In a variable-pitch propeller, the combination of a driving-shaft, aplurality of blades carried thereby and so mounted as to be capable of angular adjustment each about its own longitudinal axis, a part on said shaft having a screw-thread of small spiral angle formed on it, a nut-member on said screw-thread, means connecting said nutmember to said blades to turn them each about its own axis by a movement of the nut-member lengthwise of the operatingshaft, a power-unit for rotating said driving-shaft, a gear-wheel fast on said nutmember, two rotatable shafts carried by said nut-member and situated eccentrically with respect to the driving-shaft, a pinion on each ofvsaid shafts engaging said gear-v wheel, a second pinion on each of said shafts, two friction-wheels each operatively engaged with a gear-wheel which last-mentioned gear-wheels respectively engage said secondmentioned pinions, and a friction-element carried by said power-unit and movable at will to engage said friction-Wheels alternatively.

8. In a variable-pitch propeller, the combination of a driving-shaft, a plurality of blades carried thereby and so mounted as to be capable of angular adjustment each about its own longitudinal axis, a part on said shaft having a screw-thread of small spiral angle formed on it, a nut-member on said screw-thread,means connecting said nut-member to said blades to turn them each about its own axis by a movement of the nut-member lengthwise of the operatingshaft, a power-unit for rotating said driving-shaft, a gear-wheel fast on said nutmember, two rotatable shafts carried by said nut-member and situated eecentrically with respect to the driving shaft, a pinion on each of said shafts engaging said gear-Wheel, a second pinion on each of said shafts, two

friction-wheels each operatively engaged with a gear-wheel which last-mentioned gear-wheels respectively engage said secondmentioned pinions, and a friction-element carried by said power-unit and movable at will toengage said friction-wheels alternatively, said gears being so disposed that the movement of the nut-member resulting from the engagement of the frietion-elenlent aforesaid with one of said frietiou-wheels carries the nut-member and gearing aforesaid longitudinally of said driving-shaft in such direction as to disengage the operative friction-wheel from the friction-element aforesaid. i

In testimony whereof we have signed our names to this specification in the presence of two subscribing witnesses.

ALFRED HUBERT ROY FEDDEN. LEONARD FREDERICK GEORGE BUTLER. \Vitnesses LESLIE '1. Pon'ron, Jon P. CROCKETT. 

